1. Field of the Invention
This invention relates to semiconductor switching circuits of the variety that switch high-voltage alternating currents. More particularly, this invention relates to telephone ringing circuits which control the conduction of ringing signals to individual subscriber stations.
2. Description of the Prior Art
It is well known that semiconductor switching devices, particularly bipolar transistors, have limited applicability in high-voltage switching circuits. Excessive collector-emitter voltages can cause collector-emitter conduction without base drive; a destructive operating mode known as punch-through. It is also known that pnpn devices can safely sustain across their anode and cathode terminals greater voltages than the punch-through voltage levels of transistors.
Generally, semiconductor pnpn devices can be viewed as operating in any of four progressively increasing anode-cathode voltage regions. In a first region wherein anode-cathode voltage is well below the maximum instantaneous voltage which will not switch the device into the "on" state without gate drive, i.e., forward blocking potential, anode-cathode conduction can be initiated only upon the application of adequate gate drive. Conduction of current through the device in this first operating region is below the device's holding level and thus conduction is extinguished upon the discontinuation of the gate drive signal. In a second operating region wherein anode-cathode voltage is below the forward blocking potential of the device, but greater than that applied in the first region, conduction of current through the device is initiated upon the application of adequate gate drive. In this operating region, however, discontinuation of the gate drive will not extinguish the current through the device, as the current therethrough is greater than the device's holding current level. Anode-cathode current is extinguished by an externally-induced interruption of the current, and the device will remain off if the gate drive has been discontinued.
A third operating region is defined by anode-cathode voltage in excess of the forward blocking potential, but less than the device-destructive peak forward voltage (PFV) rating of the device. Application of such a voltage causes nondestructive conduction of anode-cathode current without external gate drive. The fourth and final operating region is defined by anode-cathode voltages in excess of the PFV; destructive conduction ensues. In both the third and fourth operating regions conduction occurs irrespective of the absence of gate control signals, and, therefore, operation in these regions is unsuitable for controlled switching functions.
The pnpn devices employed in this invention and in the relevant prior art are operated in second operating region. Control signals applied to the gate of the device effectively control the initiation of conduction, but once the device is in conduction it cannot be extinguished by gate control alone. Some means external to the device must be employed to reduce the current to a level below the device's minimum holding level. In essence, the device is extinguishable when it is biased in such a manner as to cause it to operate in the first operating region described hereinabove.
Thus, it is apparent that a first task facing the designer of switching circuits employing pnpn devices lies in solving the shut-off problems associated with these devices as a result of the "latching" effect inherent in the second operating region. The prior art has thrust at the problem with two circuit design techniques, each employing similar circuit structures but differing operative principles. The first method employs a second switching device, illustratively a bipolar transistor, in series combination with the pnpn device. By conventionally shutting off the bipolar transistor, the current through both itself and the serial pnpn device is thereby extinguished. This method, however, has the disadvantage of applying the full supply potential across the open bipolar transistor, thereby limiting the supply voltage to not exceed the transistor's punch-through level. In a more sophisticated manner, the second method employs the series bipolar transistor as a biasing agent. Conventionally signalling the bipolar transistor to begin to shut off causes the anode-cathode current through the pnpn device to diminish and the gate-cathode terminals to reverse bias. The combined effect of the diminished current and reverse-biased gate causes the pnpn device to extinguish itself not only before the bipolar transistor completely shuts off, but also before the voltage across the bipolar transistor reaches the punch-through level.
A second problem facing semiconductor switching circuit designers is that of switching unrectified alternating currents. Pnpn devices operating in the second region behave in several ways like conventional pn diodes. Conventional current is conducted in the forward-biased, anode-cathode direction, but not in the reverse-biased, cathode-anode direction. Full-wave alternating current switching has heretofore been relegated to electromechanical relays, bidirectional semiconductor devices such as triacs, and circuit schemes containing multiple pnpn devices.
It is, therefore, an object of this invention to improve solid-state switches employing a single pnpn device in series with a load for switching cyclically varying currents.
It is another object of this invention to switch a cyclically varying signal of sufficiently elevated amplitude that device-destructive collector-emitter conduction without base drive would be induced in bipolar transistors were the cyclically varying signal applied directly across the collector and emitter terminals.
It is a further object of this invention to switch off and thereby discontinue a high-voltage, cyclically varying current within a fraction of a cycle after the time that a discontinuation command signal is provided.